Infrared modulator for spectrometer

ABSTRACT

The invention relates to an infrared modulator for spectrometer. It comprises a light source ( 1 ), a beam splitter ( 2 ) for splitting the light from the source into two beams, a first plane mirror ( 4 ) for directing the light of the first beam, a second plane mirror ( 5 ) for directing the light of the second beam, a first cube corner mirror ( 7 ) for turning the light back to the first plane mirror ( 4 ), and a second cube corner mirror ( 6 ) for turning the light back to the second plane mirror ( 5 ). The first and second cube corner mirrors ( 6, 7 ) are arranged on a common optic axis to reflect into opposite directions and movable back and forth in the direction of said optic axis. According to the invention, the beam splitter ( 3 ) and the first and second plane mirrors ( 4, 5 ) are supported by the same support structure formed by one uniform material piece.

BACKGROUND OF THE INVENTION

The invention relates to an infrared modulator for spectrometer, whichcomprises a light source, a beam splitter for splitting the light fromthe source into two beams, a first plane mirror for directing the lightof the first beam, a second plane mirror for directing the light of thesecond beam, a first cube corner mirror formed of three mirrorsperpendicular to each other for turning the light directed by the firstplane mirror back to the first plane mirror, and a second cube cornermirror formed of three mirrors perpendicular to each other for turningthe light directed by the second plane mirror back to the second planemirror, the first and second cube corner mirrors being arranged on acommon optic axis to reflect into opposite directions and movable backand forth in the direction of said optic axis. The mirror system formedby the cube corner mirrors is a double cube corner whose task is toreturn the light directed to it exactly to its incoming direction to theabove-mentioned first and second mirrors and through them to the beamsplitter in which the light beams interfere with each other.

The invention is intended for use especially in a Fourier transformationspectrometer of the infrared or near-infrared regions to modulateradiation into a format that allows the calculation of the spectraldistribution of the radiation to be measured by using Fouriertransformation.

When the path lengths of the two light beams are exactly the same, aninterference maximum of all wavelengths is detected in the output of thedevice. When moving the double cube corner in the direction of the lightbeams coming from the first and second mirrors, the wavelengthdistribution of the light passing through the device can be measuredutilizing the interferences of different wavelengths.

In Fourier transformation infrared (FTIR) spectroscopy, many kinds ofdevices are used in modulating infrared radiation, the simplest beingthe Michelson interferometer based on the use of plane mirrors. Inspectroscopic applications, it is very important that the movement ofthe mirror generating modulation does not cause changes in the alignmentof the beams. This problem has been solved for instance by using what isknown as the dynamic alignment system, in which the alignment of themirror of one of the beams of the interferometer is changed continuouslyso as to maintain the modulation unchanged. Attempts have also been madeto change the movement of the mirrors such that it does not causechanges in modulation. This has been attempted for instance by using arotational movement instead of a linear movement.

The problems of alignment caused by plane mirrors have also been solvedby replacing the plane mirrors with cube corner mirrors in the Michelsoninterferometer, but these have not been able to achieve a sufficientlystable structure for field conditions. The double-beam interferometersdescribed in U.S. Pat. Nos. 4,165,183 and 4,319,843 are also based onthe use of cube corner mirrors, but neither of them is designed forFourier transformation spectroscopic applications and, thus, theirsolutions for the moving mechanism, for instance, do not serve thepurpose of the present case.

A focusing interferometer, as known for instance from U.S. Pat. No.5,459,572, has nearly achieved the desired field usability, but thesolution has, in practice, proven sensitive to vibration, because thespherical mirrors used as end mirrors are unavoidably massive and, thus,susceptible to mechanical disturbance.

SUMMARY OF THE INVENTION

An object of the present invention is to act as an instrument modulatingthe radiation of a low-resolution Fourier transformation spectrometerthat is suited for use in problematic conditions: in varyingtemperatures and vibration. This is achieved by the structure of theinvention that is characterized in that the beam splitter and the firstand second plane mirrors are supported by the same support structureformed by one uniform material piece.

The support structure supporting the beam splitter and the first andsecond plane mirrors is most preferably only fastened to its mountingplatform in one limited area of the support structure.

The best operation is achieved by a symmetrical support structure inwhich the beam splitter is on the symmetry axis and the first planemirror and the second plane mirror are symmetrically on different sidesof the beam splitter equidistant from it and the area where the supportstructure is fastened to the mounting platform is at the beam splitter.It is also advantageous to make the support structure of a homogenousand massive material, such as brass.

According to the basic idea of the invention, the desired end-result,i.e. a structure enduring varying temperatures and vibration, isachieved by taking into consideration two important factors: symmetryand a compact structure.

A symmetric structure is achieved by placing the locations of the firstand second mirrors after the beam splitter symmetrically equidistantfrom the beam splitter. When the mirror locations are also on the samepiece with the beam splitter, separate from the rest of the equipment,it is possible to ensure that temperature changes cannot affect thedifferent branches of the beam in different manners, for instancethrough the mounting platform. Because the path length difference isfourfold in comparison with the physical movement of a moving cubecorner system, even a small movement achieves sufficient resolution. Thestructure of the modulator can be made extremely compact. Further, theoptic can be implemented so that the beam splitter and the mirrorsreturning the beam, in this case a double cube corner system, are inimmediate proximity to each other, whereby the device becomes verystable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means ofpreferred embodiments and with reference to the attached drawings, inwhich

FIG. 1 is a schematic view of the operation of an infrared modulator ofthe invention,

FIG. 2 is a perspective view of a preferred embodiment of a supportstructure of mirrors used in an infrared modulator of the invention, and

FIG. 3 is a cross-section of the support structure of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 of the attached drawing is a schematic view of the generaloperation of an infrared modulator of the invention. In FIG. 1,reference number 1 is used for a light source that directs light througha plane mirror 2 to a beam splitter 3. The beam splitter splits thelight from the mirror 2 into two beams, of which one passes directlythrough the beam splitter and the other reflects from the beam splitter.The beam of light passing directly through the beam splitter is receivedby a plane mirror 5 and the beam of light reflecting from the beamsplitter is received by a plane mirror 4. The plane mirror 4 directs thelight to a cube corner mirror 7 formed by three mirrors perpendicular toeach other, which is arranged to turn the light back exactly to itsincoming direction. In the same way, the light reflected by the planemirror 5 is turned back exactly to its direction of travel by means of asecond cube corner mirror 6 formed by three mirrors perpendicular toeach other.

The cube corner mirrors 6 and 7 are arranged as close to each other aspossible on a common optic axis to reflect in the opposite directions.The cube corner mirrors 6 and 7 thus form a double cube corner whosetask is to return the incoming light exactly into its incomingdirection, i.e. back to the mirrors 4 and 5 and through them to the beamsplitter 3, where the light beams interfere with each other. With theplane mirrors 4 and 5 equidistant from the beam splitter 3 and the cubecorner mirrors equidistant from the plane mirrors 4 and 5, the pathlengths of the light beams are exactly the same, and an interferencemaximum of all wavelengths is detected in the output of the device. Thisoutput is obtained from a plane mirror 8 that receives the light fromthe beam splitter and directs the light beam outside the infraredmodulator. When the double cube corner is moved along its optic axis,i.e. back and forth in the direction of the light beams incoming fromthe plane mirrors 4 and 5, the wavelength distribution of the lightpassing through the device can be measured utilizing the interferencesof different wavelengths. The system of FIG. 1 thus forms a modulatorthat can be used in a spectrometer. Such a modulator can modulate theradiation to be measured into a format, from which the spectraldistribution of the radiation can be calculated by means of Fouriertransformation.

In accordance with the present invention, an infrared modulator isprovided, which is less sensitive than before to temperature changes andvibration. The most important reason for this is the structure, by meansof which the components of the infrared modulator are connected to eachother. Specifically, it is the support structure, which binds the beamsplitter 3 and the plane mirrors 4 and 5 together and is according tothe invention implemented as a uniform material piece and made of ahomogeneous and massive material, such as brass.

A preferred embodiment of the support structure of the invention isshown in perspective in FIG. 2. In FIG. 2, the support structure 9 thatconnects the beam splitter 3 and the plane mirrors 4 and 5 firmlytogether is shown slanted from below so that the area where it isfastened to the mounting platform is visible and marked with referencenumber 10. The part of the support structure located above the mountingplatform and generally in the shape of parallelepipeds and marked withreference number 11 is intended for fastening the beam splitter 3. Thebeam splitter 3 is fastened to an opening in the part 11 in a knownconventional manner such that it can be positioned exactly.

The support structure 9 forms, when viewed from above, a T-shaped piece,in which the foot of the T is, however, relatively short and formed ofthe above-mentioned part 11, which is for fastening the beam splitter 3.This shape of the support structure is best seen in FIG. 3. On thecrossbeam of the T, marked with reference number 12 in FIG. 2, there arebranches 13 and 14 substantially parallel with the part 11 and extendingaway from ends of the beam 12. These branches 13 and 14 are intended forfastening the plane mirrors 4 and 5. These branches 13 and 14 haveopenings, to which the plane mirrors 4 and 5 are fastened byconventional means enabling their exact positioning. So as to allowlight beams to travel unhindered from the beam splitter 3 to the planemirrors 4 and 5 fastened to the openings of the branches 13 and 14, thepart 12 of the support structure 9 is shaped in the manner shown in FIG.2 such that the generally slab-shaped piece formed by the part 12 isopen in the middle.

As shown in FIG. 1, the double cube corner system formed by the cubecorner mirrors 6 and 7 is intended for placement between the planemirrors 4 and 5. FIGS. 2 and 3 also show that this is possible inpractice. The opening in the piece 12 and placing the plane mirrors 4and 5 on the branches 13 and 14 make it possible to place the doublecube corner between the plane mirrors 4 and 5 such that it can be movedback and forth without hindrance from the support structure 9.

As shown in FIGS. 2 and 3, the support structure combining the beamsplitter 3 and the plane mirrors 4 and 5 is made of one uniform materialpiece. To obtain the best possible vibration resistance, the material ofthe support structure 9 is homogeneous and massive, such as brass. Thesupport structure 9 binds the beam splitter 3 and the mirrors 4 and 5 toeach other such that they cannot under any circumstance move relative toeach other. The support structure 9 is also symmetrical such that thebeam splitter 3 is on its symmetry axis and the mirrors 4 and 5 aresymmetrically on different sides of the beam splitter 3 equidistant fromit.

A very essential factor in the infrared modulator of the invention isalso that the support structure is fastened to its mounting platform,such as modulator frame or casing, in only one limited area. This areais marked with reference number 10 in FIG. 2. It is on the symmetry axisof the support structure below the beam splitter 3. As shown in FIG. 2,the area 10 is equipped with three support pins 15 having centraldrillings 16 with inner threads. Thus, the support structure 9 can befastened to the mounting platform by inserting three screws through itthat penetrate the drillings 16 of the support pins 15 and fasten thesupport structure 9 to the platform. This way, the support structure isonly supported at one point that is symmetrically located with respectto the support structure. This type of support method is advantageous inview of both the thermal expansion and vibration resistance of thesupport structure 9. When using this type of support method, a possiblethermal expansion of the support structure 9 in connection withtemperature changes affects evenly the entire structure and thesymmetrical dimensions of the support structure 9 remain unchanged.

Since this invention does not concern the moving mechanism of the doublecube corner system 6, 7, it is not described in more detail. However,the following examines the requirements set for this moving system andits possible implementation methods based on the prior art.

Moving the double cube corner system 6, 7 that returns beams requiresthat the movement remain exactly on the optic axis of the mirrors andespecially that the positions of the corners of both halves of themirror system remain exactly the same with respect to the optic axis. Infield conditions susceptible to vibration and temperature changes, thisis a difficult requirement, since the device should also be compact.Thus, the busbar-controlled belt-driven moving mechanism according toU.S. Pat. No. 4,165,183 is not suitable, since it is mainly intended forlaboratory conditions, but a substantially better solution is disclosedin U.S. Pat. No. 5,457,531. In it, the mechanism that moves the mirrorsystem returning beams is a pendulum-type mechanism having at least twoflexible arms, with which the movement of the mirror system returningbeams can be made linear in the desired movement range. The movingmechanism known from said publication is incorporated herein byreference.

The infrared modulator of the invention is above described by means ofonly one exemplary embodiment. It is obvious to a person skilled in theart that the basic idea of the invention can be implemented in manydifferent ways. The invention and its embodiments are thus notrestricted to the example described above, but can vary within the scopeof the claims.

1. An infrared modulator for spectrometer, comprising a light source; abeam splitter for splitting the light from the source into two beams; afirst plane mirror for directing the light of the first beam; a secondplane mirror for directing the light of the second beam, a first cubecorner mirror formed of three mirrors perpendicular to each other forturning the light directed by the first plane mirror back to the firstplane mirror; a second cube corner mirror formed of three mirrorsperpendicular to each other for turning the light directed by the secondplane mirror back to the second plane mirror, the first and second cubecorner mirrors being arranged on a common optic axis to reflect intoopposite directions and movable back and forth in the direction of saidoptic axis; and a support structure formed by one uniform material piecefor supporting the beam splitter and the first and second plane mirrors.2. An infrared modulator as claimed in claim 1, wherein the supportstructure is symmetrical such that the beam splitter is on its symmetryaxis and the first and second plane mirrors are symmetrically ondifferent sides of the beam splitter equidistant from it.
 3. An infraredmodulator as claimed in claim 1, wherein the support structure isprovided with only in one limited area for fastening the supportstructure to a mounting platform.
 4. An infrared modulator as claimed inclaim 3, wherein said one limited area for fastening the supportstructure to a mounting platform is at the beam splitter.
 5. An infraredmodulator as claimed in claim 1, wherein the support structure is madeof a homogenous and massive material, such as brass.